Security Circuits

The resistor network at pins 6 and 7 of the LM555 is incorrect. Instead of a fixed 200k and a 500k pot in parallel, it should be the 200k fixed in parallel with a 500k pot and the resulting two in series with a 1k fixed. Thus the 200k fixed established the maximum duty cycle, and the 1k pot establishes the minimum duty cycle. You could try eliminating the 1K, but you might damage the LM555...

This circuit features automatic Exit/Entry delays, timed Bell Cut-off and System Reset. It has provision for normally open and normally closed switches and will accommodate the usual input devices such as Foil Tape, Pressure Mats, Magnetic Reed Contacts, Passive Infrared Detectors and Inertia (Shock) Sensors...

The thermistor used has a resistance of 15k at 25 degrees and 45k at 0 degrees Celsius. A suitable bead type thermistor can be found in the Maplin catalogue. The 100k pot allows this circuit to trigger over a wide range of temperatures. If using a different thermistor then the control should match the new thermistor at its highest resistance, or be higher in value. The op-amp in this circuit is the ubiquitous 741...